Transmissão em Corrente Contínua

Transcription

1 Transmissão em Corrente Contínua Panorama Atual e Perspectivas Futuras no Brasil

2 Multi-Terminal HVDC Classic Some Considerations Brazilian SC B4 Paulo Fischer de Toledo, ABB

3 Basic considerations Traditional 2-terminal control at converter level Many operation modes become possible (In 2-terminal case, only bipolar and monopolar) Somewhat larger dimensioning of main circuit Additional functions needed in the control

4 The technology is not new 1963 paper on tapping (Uno Lamm et al) 1978 analog simulator study (Göte Liss et al) 1989 Italy Sardinia adds tap in Corsica Québec New England. The first large scale multi-terminal transmission system Paralleling used in Inga-Shaba Itaipú Nelson River North East Agra. The newest large scale multi-terminal transmission system

5 The SACOI transmission - Italy Sardinia Corsica Italia By Ansaldo / GE and CGEE.Alsthom Supply to both islands Sardinia 300 MW Corsica 50 MW Mainland 300 MW

6 Québec New England (QNE) - Canada

7 Québec New England (QNE) Started as 2-terminal transmission (Des Cantons Comerford) Went on to 5-terminal (Radisson, Nicolet and Sandy Pond added) Went on to 3-terminal (The original stations were decommissioned) Technical Data: In operation: DC Voltage: +/- 450 kv DC Line: 1480 km Power Rating: Radisson 2250 MW; Nicolet 2138 MW; Sandy Point 1800 MW

8 North East Agra (India) Indian Power Systems

9 Multi-terminal HVDC classic, configurations Bipolar Multiterminal 12-pulse groups Hybrid Multiterminal Metallic Return 12-pulse groups

10 Tapping of HVDC

11 Québec New England (QNE) DC switching arrangement with switching type

12 Québec New England (QNE) DC switching arrangement with switching type High speed disconnect switches Combine the following functions High speed converter isolation DC line sectionalizing DC transfer switches Circuit breaker with full DC current commutating capability used for converter DC neutral side isolation For metallic return transfer For ground return transfer Motor operated disconnect switches used for Isolation Connecting the metallic return path Polarity reversal

13 Québec New England (QNE) DC switching arrangement Each configuration allows bipolar/monopolar operating modes with/without metallic return Multi-terminal configurations allows hybrid bipolar monopolar metallic return

14 Basic control principles The current margin control principle is used as for a conventional 2-terminal system One terminal must be appointed as Voltage Setting Terminal (VST) Characteristics below assume DC line resistance neglected Ud Ud Ud Ud Id Id Id Id Id

15 Basic control principles Voltage control All stations are in parallel Only one station is allowed to control the voltage (VST = Voltage Setting Terminal) The VST is normally a large inverter in a large ac system The Master Controller orders the voltage reference at the VST The Master Controller checks that all stations are within tolerances

16 Basic control principles Current control All stations are in parallel All stations but one (VST) can control their currents The VST is normally a large inverter in a large ac system The Master Controller orders the power references

17 Control: Some additional control functions for multi-terminal Current order balancing Voltage control (not really additional, but different) Stabilization/modulation control (not really additional, but different) Power order ramping coordination Post fault recovery & reconfiguration Possible faulted areas/components analysed before-hand, as well as the best post-fault configurations The Master Controller calculates best load flows and power orders The Master Controller suggests configuration(s) and power order(s) The Master Controller executes, if operator accepts suggestions

18 Master Control Simplified Block Diagram, Multi-Terminal Power Control

19 Master Control Some key functions part 1 Balancing the Power Order Sum of Power Orders to all involved rectifies is equal to the sum of the Power Orders to all involved inverters Weighting factors C1-C3 are set by operators

20 Master Control Some key functions part 2 Coordinate the power order ramping Load flow change may result the involvement from more than two converters ramping speed must be coordinated All stations that are designated to contributed with power must be ramped accordingly to reach the new power at the same time

21 Master Control Some key functions part 3 Current Order calculation Current order calculation is performed by dividing balanced power order by direct voltage Voltage measurement values are transmitted from converter stations to Master Control Voltage measurement is smoothed by long time-constant to avoid stability problems if converters are connected to weak AC system

22 Master Control Some key functions part 4 Compensation for DC line losses Due to losses in the DC line current order are unbalanced even if Power Orders are balanced The sum of current order with different signals (for rectifier or inverter operation) is added to one of Current Order (with suitable signal) and the losses will be compensated in that station

23 Master Control Some key functions part 5 Balancing taking maximum limitations into consideration Sum of current orders coming from all rectifiers must be equal to the sum of current fed into all inverters A current order balancing function is needed taking into account the maximum limitations for current orders By the weighting factors K1-K3 and the feedback loop, an unbalance in input Current Order is eliminated so that the output orders are balanced

24 Master Control Some key functions part 6 Coordination of Power Modulation Different projects require different Power Modulation Master Control can activate/deactivate specific modulating function with/without intervention of the operator Typical Modulating Functions: Frequency Control for generators (case of isolated system) Network stabilization for specific network conditions Damping Control in case of inter-area oscillations

25 Master Control Some key functions part 7 Voltage Control DC system voltage is determined by one converter station (VST Voltage Setting Terminal) VST station is the inverter with highest power and operating in local feedback voltage control mode. Master Control supervises the voltage profile of the DC system and may order a reduction of the voltage reference in the VST if voltage in any station tends to be too high

26 Master Control Some key functions part 8 Post fault recovery and re-configuration Loss of a converter or Line section the remaining parts of the affected pole can often still be operated current orders are then re-arranged to obtain balance values For loss of converter Current Order are rearranged by balancing function in accordance with the weighting factors K1-K3 Master Control can assist the operator in making a decision for a new configuration if is pre-programmed for such assignment

27 Master Control Telecommunication part 1 Telecommunication (Control and Protection) of a Multi-Terminal is more complex than a 2-terminal system All the stations must have a direct communication to the Master Control For reliability reasons the communication channels are duplicated

28 Master Control Telecommunication part 2 There are two different types of redundant telecommunication links The Master Telecommunication (MTCOM) transmits information such as: Current Order set-point Current Order limit Orders Status of Master Control and each converter on a per pole basis

29 Master Control Telecommunication part 3 Protective telecommunication (PTCOM) transmits protective orders from faulted converter directly to all other converters: Force retards Block signals Line protective inter-lockings

30 Master Control Telecommunication part 4 Current Order Synchronization New current order (ramping, limitations, modulations) are synchronized to prevent loss of current margin Synchronization consists of transmission of current orders with checkback for security Synchronization logic differs for current order increase or decrease to avoid reduction in the current margin Trip of converter new current order are transmitted without synchronization during isolation time for maximum speed

31 Master Control Telecommunication failure (Local Control) part 1 Absence of communication Back-up controls are activated locally Back-up controls allows: Starting of converter Stopping of converter Power change Frequency Control modulation Re-allocation of power following forced outage However Operator flexibility is reduced Slow recovery from certain system faults (typically DC line faults)

32 Master Control Telecommunication failure (Local Control) part 2 Current Order Memory Normal operation Current Order Memory fallows thr master current order During telecommunication failure the current Order Memory is in effect and can be changed by the operator Frequency control modulation is allowed to operate but has to preserve the current margin Current order in the VST tracks the current response except during disturbances like faults or mode shifts

33 Master Control Telecommunication failure (Local Control) part 3 Decentralized Dynamic Voltage Controlled Recovery (DDVCR) Purpose: Case of converter trip without MTCOM Avoid possible overload Preserve the Current Margin Ensure an active VST

34 Conclusions Multi-terminal configurations in HVDC are not new The technology exists and has been time tested The main circuit equipment can become marginally more expensive The control becomes, internally, more complex, but the operator gets more help from the control system.

35 Multi-terminal Challenges: Economical HVDC lines are less expensive than AC, This pays for terminals being more expensive Multi-terminal adds on the expensive side Converter stations for multi-terminal are more expensive than those for two-terminal operation: Larger steady state gamma and thereby Larger Udio More reactive power More switchyard equipment (disconnects, HSS s, DCCT s )

36 Multi-terminal Challenges: Control considerations Master controllers One in each station All are exact replicas, and kept live and updated Only one is in charge These principles are the same as in 2-terminal BUT There are more master controllers They are more complex and contain more functions Telecommunication plays an important role, but system can operate without it